It does not base on the idea of the lore being a pinnacle, it is based on the idea of the lore being the origin of which any adaptation is considered a copy. And any alteration can be (subjectively) considered a diminished copy of the original, it can be considered to be a bastardization.
Now, whether anyone consider its to be a bastardization is and can be subjective. However it doesn't mean it's ONLY subjective, since like the term 'Good' can be used subjectively, it can also be applied objectively when referring to a specific instance, like '90% score is typically considered a Good score'; this is not a subjective value this is an objective application of the term. And if we're looking at how of Rings of Power has transformed the lore of the original to suit its own narrative, then the term 'Lore bastardization' can be objectively applied, because bastardization means a corruption of the original. And I have brought up many cases where RoP CAN BE CONSIDERED corrupting the original lore.
You can disagree with the entire premise if you want, and it would be on a subjective premise that you don't agree that the lore should be considered in such a way; but you can't really dismiss the nature of the term being applied objectively to describe the relationship between the adaptation and the original source. The term is being applied the way it's meant to be used.
That you don't think the lore should be given any appeal to authenticity is a completely different argument than the one I'm making about the objective application of the term, much in the way 'Good score' would be objectively applied outside of any subjective context. 'Lore Bastardization' means a diminished copy of the original history, and that is an objective term that is (subjectively) applicable to RoP's adaptation of the original Book lore. Much in the way 'Good Score' is term that is both applicable objectively and subjectively.